[ELPA] New package: dired-du

[Tino Calancha]

New library implementing a Dired interface for `du'.

I)
This library defines a minor mode `dired-du-mode' to display
the recursive size of directories in Dired buffers.  This mode
uses the external `du' program when available.  Otherwise, it
performs a rough estimation with Lisp.

II)
In addition, this file provides a command `dired-du-count-sizes',
to show the total size of the marked files.  By default, it shows
the size of the files marked with `dired-marker-char'.   If `dired-du-mode'
is disabled, then ignores the size of directories.  Otherwise, it takes
in account the size of the dirs.
When called with a prefix prompts for the marks and asks if the dirs must
be taken in account.

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The idea of this library arose after bugs 22829 and 22892.

Some people have shown interest on this feature elsewhere:
https://emacs.stackexchange.com/questions/20766/display-recursive-folder-sizes-in-line-in-dired
https://superuser.com/questions/462079/how-can-i-display-folder-sizes-in-emacs-dired

The file is available for inspection in the following Elpa repo. branch:
scratch/dired-du